Advances in technology have resulted in smaller and more powerful computing devices. For example, there currently exist a variety of portable personal computing devices, including wireless computing devices, such as portable wireless telephones, personal digital assistants (PDAs), tablet computers, and paging devices that are small, lightweight, and easily carried by users. Many such computing devices include other devices that are incorporated therein. For example, a wireless telephone can also include a digital still camera, a digital video camera, a digital recorder, and an audio file player. Also, such computing devices can process executable instructions, including software applications, such as a web browser application that can be used to access the Internet and multimedia applications that utilize a still or video camera and provide multimedia playback functionality.
To support such advanced functionality, the computing devices may include an operating system. The operating system may provide abstractions of available hardware resources. For example, an operating system may multiplex system resources (e.g., processor, memory, etc.) amongst various software tasks. The operating system may also schedule tasks for execution and perform memory management. In addition, the operating system may handle blocking and unblocking for events.
In certain situations, it may be advantageous to execute multiple operating systems, or multiple instances of the same operating system, at a single computing device. A hypervisor (also known as a virtual machine monitor) may provide abstractions of available hardware resources to the operating systems. Thus, the hypervisor may act as an operating system for the multiple operating systems (or multiple instances of the same operating system). When a hypervisor is used, the hypervisor may include a scheduler to determine which operating system and task thereof is granted access to a particular hardware resource at a particular time. Thus, multiple levels of scheduling may be executing within the system: a first level may include the schedulers of each individual operating system and a second level may include the scheduler of the hypervisor.
Performing multiple levels of scheduling introduces overhead into the system (e.g., due to context switches performed by the hypervisor). Such overhead may be unacceptable when one or more of the operating systems is a real-time operating system that needs to be able to guarantee certain latencies. To include a real-time operating system in a multiple operating system environment, the real-time operating system is usually given “special” priority. For example, the real-time operating system may have a highest available priority. However, this may be inefficient because task priority information within each operating system may not be accessible to the hypervisor. For example, if the real-time operating system is executing an unimportant task but another operating system is executing an important task, the hypervisor may still schedule the relatively unimportant real-time operating system task first.